root/init/calibrate.c

DEFINITIONS

1. lpj_setup
2. calibrate_delay_direct
3. calibrate_delay_direct
4. calibrate_delay_converge
5. calibrate_delay_is_known
6. calibration_delay_done
7. calibrate_delay

   1 // SPDX-License-Identifier: GPL-2.0
   2 /* calibrate.c: default delay calibration
   3  *
   4  * Excised from init/main.c
   5  *  Copyright (C) 1991, 1992  Linus Torvalds
   6  */
   7 
   8 #include <linux/jiffies.h>
   9 #include <linux/delay.h>
  10 #include <linux/init.h>
  11 #include <linux/timex.h>
  12 #include <linux/smp.h>
  13 #include <linux/percpu.h>
  14 
  15 unsigned long lpj_fine;
  16 unsigned long preset_lpj;
  17 static int __init lpj_setup(char *str)
  18 {
  19         preset_lpj = simple_strtoul(str,NULL,0);
  20         return 1;
  21 }
  22 
  23 __setup("lpj=", lpj_setup);
  24 
  25 #ifdef ARCH_HAS_READ_CURRENT_TIMER
  26 
  27 /* This routine uses the read_current_timer() routine and gets the
  28  * loops per jiffy directly, instead of guessing it using delay().
  29  * Also, this code tries to handle non-maskable asynchronous events
  30  * (like SMIs)
  31  */
  32 #define DELAY_CALIBRATION_TICKS                 ((HZ < 100) ? 1 : (HZ/100))
  33 #define MAX_DIRECT_CALIBRATION_RETRIES          5
  34 
  35 static unsigned long calibrate_delay_direct(void)
  36 {
  37         unsigned long pre_start, start, post_start;
  38         unsigned long pre_end, end, post_end;
  39         unsigned long start_jiffies;
  40         unsigned long timer_rate_min, timer_rate_max;
  41         unsigned long good_timer_sum = 0;
  42         unsigned long good_timer_count = 0;
  43         unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
  44         int max = -1; /* index of measured_times with max/min values or not set */
  45         int min = -1;
  46         int i;
  47 
  48         if (read_current_timer(&pre_start) < 0 )
  49                 return 0;
  50 
  51         /*
  52          * A simple loop like
  53          *      while ( jiffies < start_jiffies+1)
  54          *              start = read_current_timer();
  55          * will not do. As we don't really know whether jiffy switch
  56          * happened first or timer_value was read first. And some asynchronous
  57          * event can happen between these two events introducing errors in lpj.
  58          *
  59          * So, we do
  60          * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  61          * 2. check jiffy switch
  62          * 3. start <- timer value before or after jiffy switch
  63          * 4. post_start <- When we are sure that jiffy switch has happened
  64          *
  65          * Note, we don't know anything about order of 2 and 3.
  66          * Now, by looking at post_start and pre_start difference, we can
  67          * check whether any asynchronous event happened or not
  68          */
  69 
  70         for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  71                 pre_start = 0;
  72                 read_current_timer(&start);
  73                 start_jiffies = jiffies;
  74                 while (time_before_eq(jiffies, start_jiffies + 1)) {
  75                         pre_start = start;
  76                         read_current_timer(&start);
  77                 }
  78                 read_current_timer(&post_start);
  79 
  80                 pre_end = 0;
  81                 end = post_start;
  82                 while (time_before_eq(jiffies, start_jiffies + 1 +
  83                                                DELAY_CALIBRATION_TICKS)) {
  84                         pre_end = end;
  85                         read_current_timer(&end);
  86                 }
  87                 read_current_timer(&post_end);
  88 
  89                 timer_rate_max = (post_end - pre_start) /
  90                                         DELAY_CALIBRATION_TICKS;
  91                 timer_rate_min = (pre_end - post_start) /
  92                                         DELAY_CALIBRATION_TICKS;
  93 
  94                 /*
  95                  * If the upper limit and lower limit of the timer_rate is
  96                  * >= 12.5% apart, redo calibration.
  97                  */
  98                 if (start >= post_end)
  99                         printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
 100                                         "timer_rate as we had a TSC wrap around"
 101                                         " start=%lu >=post_end=%lu\n",
 102                                 start, post_end);
 103                 if (start < post_end && pre_start != 0 && pre_end != 0 &&
 104                     (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
 105                         good_timer_count++;
 106                         good_timer_sum += timer_rate_max;
 107                         measured_times[i] = timer_rate_max;
 108                         if (max < 0 || timer_rate_max > measured_times[max])
 109                                 max = i;
 110                         if (min < 0 || timer_rate_max < measured_times[min])
 111                                 min = i;
 112                 } else
 113                         measured_times[i] = 0;
 114 
 115         }
 116 
 117         /*
 118          * Find the maximum & minimum - if they differ too much throw out the
 119          * one with the largest difference from the mean and try again...
 120          */
 121         while (good_timer_count > 1) {
 122                 unsigned long estimate;
 123                 unsigned long maxdiff;
 124 
 125                 /* compute the estimate */
 126                 estimate = (good_timer_sum/good_timer_count);
 127                 maxdiff = estimate >> 3;
 128 
 129                 /* if range is within 12% let's take it */
 130                 if ((measured_times[max] - measured_times[min]) < maxdiff)
 131                         return estimate;
 132 
 133                 /* ok - drop the worse value and try again... */
 134                 good_timer_sum = 0;
 135                 good_timer_count = 0;
 136                 if ((measured_times[max] - estimate) <
 137                                 (estimate - measured_times[min])) {
 138                         printk(KERN_NOTICE "calibrate_delay_direct() dropping "
 139                                         "min bogoMips estimate %d = %lu\n",
 140                                 min, measured_times[min]);
 141                         measured_times[min] = 0;
 142                         min = max;
 143                 } else {
 144                         printk(KERN_NOTICE "calibrate_delay_direct() dropping "
 145                                         "max bogoMips estimate %d = %lu\n",
 146                                 max, measured_times[max]);
 147                         measured_times[max] = 0;
 148                         max = min;
 149                 }
 150 
 151                 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
 152                         if (measured_times[i] == 0)
 153                                 continue;
 154                         good_timer_count++;
 155                         good_timer_sum += measured_times[i];
 156                         if (measured_times[i] < measured_times[min])
 157                                 min = i;
 158                         if (measured_times[i] > measured_times[max])
 159                                 max = i;
 160                 }
 161 
 162         }
 163 
 164         printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
 165                "estimate for loops_per_jiffy.\nProbably due to long platform "
 166                 "interrupts. Consider using \"lpj=\" boot option.\n");
 167         return 0;
 168 }
 169 #else
 170 static unsigned long calibrate_delay_direct(void)
 171 {
 172         return 0;
 173 }
 174 #endif
 175 
 176 /*
 177  * This is the number of bits of precision for the loops_per_jiffy.  Each
 178  * time we refine our estimate after the first takes 1.5/HZ seconds, so try
 179  * to start with a good estimate.
 180  * For the boot cpu we can skip the delay calibration and assign it a value
 181  * calculated based on the timer frequency.
 182  * For the rest of the CPUs we cannot assume that the timer frequency is same as
 183  * the cpu frequency, hence do the calibration for those.
 184  */
 185 #define LPS_PREC 8
 186 
 187 static unsigned long calibrate_delay_converge(void)
 188 {
 189         /* First stage - slowly accelerate to find initial bounds */
 190         unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
 191         int trials = 0, band = 0, trial_in_band = 0;
 192 
 193         lpj = (1<<12);
 194 
 195         /* wait for "start of" clock tick */
 196         ticks = jiffies;
 197         while (ticks == jiffies)
 198                 ; /* nothing */
 199         /* Go .. */
 200         ticks = jiffies;
 201         do {
 202                 if (++trial_in_band == (1<<band)) {
 203                         ++band;
 204                         trial_in_band = 0;
 205                 }
 206                 __delay(lpj * band);
 207                 trials += band;
 208         } while (ticks == jiffies);
 209         /*
 210          * We overshot, so retreat to a clear underestimate. Then estimate
 211          * the largest likely undershoot. This defines our chop bounds.
 212          */
 213         trials -= band;
 214         loopadd_base = lpj * band;
 215         lpj_base = lpj * trials;
 216 
 217 recalibrate:
 218         lpj = lpj_base;
 219         loopadd = loopadd_base;
 220 
 221         /*
 222          * Do a binary approximation to get lpj set to
 223          * equal one clock (up to LPS_PREC bits)
 224          */
 225         chop_limit = lpj >> LPS_PREC;
 226         while (loopadd > chop_limit) {
 227                 lpj += loopadd;
 228                 ticks = jiffies;
 229                 while (ticks == jiffies)
 230                         ; /* nothing */
 231                 ticks = jiffies;
 232                 __delay(lpj);
 233                 if (jiffies != ticks)   /* longer than 1 tick */
 234                         lpj -= loopadd;
 235                 loopadd >>= 1;
 236         }
 237         /*
 238          * If we incremented every single time possible, presume we've
 239          * massively underestimated initially, and retry with a higher
 240          * start, and larger range. (Only seen on x86_64, due to SMIs)
 241          */
 242         if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
 243                 lpj_base = lpj;
 244                 loopadd_base <<= 2;
 245                 goto recalibrate;
 246         }
 247 
 248         return lpj;
 249 }
 250 
 251 static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
 252 
 253 /*
 254  * Check if cpu calibration delay is already known. For example,
 255  * some processors with multi-core sockets may have all cores
 256  * with the same calibration delay.
 257  *
 258  * Architectures should override this function if a faster calibration
 259  * method is available.
 260  */
 261 unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
 262 {
 263         return 0;
 264 }
 265 
 266 /*
 267  * Indicate the cpu delay calibration is done. This can be used by
 268  * architectures to stop accepting delay timer registrations after this point.
 269  */
 270 
 271 void __attribute__((weak)) calibration_delay_done(void)
 272 {
 273 }
 274 
 275 void calibrate_delay(void)
 276 {
 277         unsigned long lpj;
 278         static bool printed;
 279         int this_cpu = smp_processor_id();
 280 
 281         if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
 282                 lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
 283                 if (!printed)
 284                         pr_info("Calibrating delay loop (skipped) "
 285                                 "already calibrated this CPU");
 286         } else if (preset_lpj) {
 287                 lpj = preset_lpj;
 288                 if (!printed)
 289                         pr_info("Calibrating delay loop (skipped) "
 290                                 "preset value.. ");
 291         } else if ((!printed) && lpj_fine) {
 292                 lpj = lpj_fine;
 293                 pr_info("Calibrating delay loop (skipped), "
 294                         "value calculated using timer frequency.. ");
 295         } else if ((lpj = calibrate_delay_is_known())) {
 296                 ;
 297         } else if ((lpj = calibrate_delay_direct()) != 0) {
 298                 if (!printed)
 299                         pr_info("Calibrating delay using timer "
 300                                 "specific routine.. ");
 301         } else {
 302                 if (!printed)
 303                         pr_info("Calibrating delay loop... ");
 304                 lpj = calibrate_delay_converge();
 305         }
 306         per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
 307         if (!printed)
 308                 pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
 309                         lpj/(500000/HZ),
 310                         (lpj/(5000/HZ)) % 100, lpj);
 311 
 312         loops_per_jiffy = lpj;
 313         printed = true;
 314 
 315         calibration_delay_done();
 316 }